1. Field of the Invention
This invention relates to improvements in electrical circuits, and, more particularly, to improvements in circuits for driving inductive loads for use in such application as dc motor drivers and the like.
2. Description of the Prior Art
In the design of motor controller circuits, typically a transconductance loop is provided that is stable for resistive loads, and that has an output current delivered to drive the motor in response to an input voltage. However, because such motors have associated inductive loads, the driver circuit loop may become unstable.
To stabilize the driver circuit loop, "snubber" networks are often employed. A typical snubber network comprises a resistor in series with a capacitor, connected in parallel with the inductive load. The effect of the snubber network can be seen in a Bode diagram as appearing resistive at higher frequencies, thereby limiting the apparent value of the impedance of the load. If the inductive pole of the load is made equal to the capacitive zero of the capacitor of the snubber, a constant resistance over frequency, in principle, might be achieved. The value of the resistor, on the other hand, may be provided in the range of tens to hundreds of ohms, so that at higher frequencies the value of the snubber resistor becomes the impedance that is seen in the load (the inherent resistance in the inductive coils being essentially negligible).
If the load is a part of the gain function, a low value of load is maintained, and, therefore, a low value of the gain function is maintained. Thus, a snubber circuit avoids an excessive increase of the gain of the circuit at higher frequencies. Another way of looking at the operation of the snubber is to consider the action of the resistor of the snubber circuit as reducing the Q of the coil of the motor and damping the energy stored in the inductor. It can be seen that one of the disadvantageous effects of such snubber circuits is the power dissipation required from the circuit. This power dissipation becomes especially significant in PWM type circuits due to the high currents that pass through the capacitor at the high switching frequencies employed.
The action of the snubber circuit in damping the voltage response of the load may be understood as follows. A discontinuity in the driving stage produces a significant spike in the output due to the inductive load. Thus, intrinsically a high frequency type of signal is generated at the output of the motor driver. Such high frequency components contribute significantly to the generation of electromagnetic interference (EMI). Such EMI is undesirable, especially in hard disk drive motor applications, since such high frequencies appear in the read-write channel associated with such hard disk drive.
Such snubber circuits as described above typically are used to decrease the noise content in the output between the driver circuit and the inductive load by decreasing the frequency bandwidth in which the noise occurs. Due to the intrinsic time constants of the load, the bandwidth of the loop oftentimes is in or below the audible frequency range. Since the torque of the motor is proportional to the current flowing through the motor, the ripple that is generated during transitions may produce audible noise that is oftentimes objectionable to a user. Moreover the ripple effects on the motor may also affect its precision.
In addition, typical snubber networks employ fairly large capacitors, for example, on the order of ten microfarads or higher often being used, commonly in the range of hundreds of nanofarads to microfarads. With resistors in the tens of ohms range, it can be seen that the snubber networks typically are required to employ power type components provided by discrete external component elements, and cannot be, for example, integrated into a part of a motor driver integrated circuit chip or the like. Such discrete component elements are bulky and expensive.
Another disadvantage of using snubber networks that produce significant ripple in the operation of motor drivers, particularly those used, for example, to drive computer hard disks or the like, is a reduction in the precision of the motor speed, which translates into reduced precision of the disk drive itself and the accuracy that can be achieved in inputting and outputting data to it.